Regioselective Alkylative Carboxylation of Allenamides with Carbon Dioxide and Dialkylzinc Reagents Catalyzed by an N-Heterocyclic Carbene–Copper Complex

Article abstract of DOI:10.1002/chem.201601162

The alkylative carboxylation of allenamide catalyzed by an N-heterocyclic carbene (NHC)–copper(I) complex [(IPr)CuCl] with CO₂and dialkylzinc reagents was investigated. The reaction of allenamides with dialkylzinc reagents (1.5 equiv) and CO₂(1 atm.) proceeded smoothly in the presence of a catalytic quantity of [(IPr)CuCl] to afford (Z)-α,β-dehydro-β-amino acid esters in good yields. The reaction is regioselective, with the alkyl group introduced onto the less hindered γ-carbon, and the carboxyl group introduced onto the β-carbon atom of the allenamides. The first step of the reaction was alkylative zincation of the allenamides to give an alkenylzinc intermediate followed by nucleophilic addition to CO₂. A variety of cyclic and acyclic allenamides were found to be applicable to this transformation. Dialkylzinc reagents bearing β-hydrogen atoms, such as Et₂Zn or Bu₂Zn, also gave the corresponding alkylative carboxylation products without β-hydride elimination. The present methodology provides an easy route to alkyl-substituted α,β-dehydro-β-amino acid ester derivatives under mild reaction conditions with high regio- and stereoselectivtiy.

Full text of DOI:10.1002/chem.201601162

DOI: 10.1002/chem.201601162
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Catalysis |Hot Paper|
Regioselective Alkylative Carboxylation of Allenamides with
Carbon Dioxide and Dialkylzinc Reagents Catalyzed by an
N-Heterocyclic Carbene–Copper Complex
[a, b]
[a]
[a, b]
Sandeep Suryabhan Gholap,
Masanori Takimoto,* and Zhaomin Hou*
Abstract: The alkylative carboxylation of allenamide cata-
lyzed by an N-heterocyclic carbene (NHC)–copper(I) complex
reaction was alkylative zincation of the allenamides to give
an alkenylzinc intermediate followed by nucleophilic addi-
[
(IPr)CuCl] with CO and dialkylzinc reagents was investigat-
tion to CO . A variety of cyclic and acyclic allenamides were
2
2
ed. The reaction of allenamides with dialkylzinc reagents
1.5 equiv) and CO (1 atm.) proceeded smoothly in the pres-
found to be applicable to this transformation. Dialkylzinc re-
(
agents bearing b-hydrogen atoms, such as Et Zn or Bu Zn,
2
2
2
ence of a catalytic quantity of [(IPr)CuCl] to afford (Z)-a,b-de-
hydro-b-amino acid esters in good yields. The reaction is re-
gioselective, with the alkyl group introduced onto the less
hindered g-carbon, and the carboxyl group introduced onto
the b-carbon atom of the allenamides. The first step of the
also gave the corresponding alkylative carboxylation prod-
ucts without b-hydride elimination. The present methodolo-
gy provides an easy route to alkyl-substituted a,b-dehydro-
b-amino acid ester derivatives under mild reaction condi-
tions with high regio- and stereoselectivtiy.
Introduction
of functionalized allenes, have recently received much atten-
tion in the synthetic community. In this regard, various syn-
thetic methods to prepare nitrogen containing compounds,
such as N-heterocycles, from allenamides have been devel-
The development of both green and sustainable strategies for
organic synthesis has received increasing attention from both
[
8]
academia and industry. In this context, carbon dioxide (CO ) is
oped. The carboxylation of allenamides with CO would give
2
2
an attractive C1-building block in organic synthesis because it
is an abundant, inherently renewable, and an environmentally
the nitrogen-substituted a,b-unsaturated carboxylic acid deriv-
atives known as a,b-dehydro-b-amino acids. These a,b-dehy-
dro-b-amino acid derivatives have many potential applications
[1]
benign carbon resource. Hence, development of new meth-
ods to utilize CO2 for the synthesis of organic compounds
through carbon–carbon bond-forming processes is currently
an important research subject. In particular, exploration of new
[
9]
as precursors of b-amino acids, terpene alkaloid derivatives,
[
10]
such as Gentiocrucine, and other classes of valuable com-
[
11]
pounds. When a,b-dehydro-b-amino acids are incorporated
into peptides, it can often induce a constricted conformation
of the corresponding peptide leading to more favorable bind-
catalytic systems that enable CO incorporation into organic
2
compounds under mild reaction conditions plays a central role
[
12]
in advancing the science of CO utilization. To date, various or-
ing of target molecules. Therefore, the synthesis of a,b-dehy-
dro-b-amino acids has gained significant attention and various
2
[
2,3]
[4]
[5]
ganic substrates such as alkynes,
dienes, enynes, and al-
[
6,7]
[13]
lenes have been employed in CO incorporation reactions as
methods have been reported. For instance, Helquist report-
2
such reactions give unsaturated carboxylic acids and deriva-
tives that are an important class of compounds in organic syn-
thesis. Among the various potentially interesting unsaturated
ed the tin-promoted addition of malonate to simple nitriles to
[
14]
give a,b-dehydro-b-amino acid derivatives. Jia and co-work-
ers reported the synthesis of a,b-dehydro-b-amino esters from
[
15]
substrates for CO incorporation reactions, allenamides, a class
Baylis–Hillman acetates with primary amines. Iqbal and co-
workers also independently reported the Pd-catalyzed reaction
of primary amines with Baylis–Hillman acetates for the synthe-
2
[
16]
[
a] S. S. Gholap, Dr. M. Takimoto, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory and Center for Sustainable
Resource Science, RIKEN, 2-1 Hirosawa, Wako
Saitama 351-0198 (Japan)
sis of a,b-dehydro-b-amino acid derivatives.
Although the
carboxylation of allenamides can be seen as a straightforward
route for the synthesis of a,b-dehydro-b-amino acid deriva-
tives, the catalytic incorporation of CO2 into an allenamide
compound has not been reported previously. Recently, Sato
and co-workers reported a nickel-mediated carboxylation of al-
E-mail: houz@riken.jp
mtakimoto@riken.jp
[
b] S. S. Gholap, Prof. Dr. Z. Hou
Graduate School of Science and Engineering
Saitama University, 255 Shimo-okubo, Sakura-ku
Saitama 338-8570 (Japan)
lenamide with CO , which gave a,b-dehydro-b-amino acid de-
2
[
17]
rivatives.
In this case, a stoichiometric quantity of the
nickel(0) species was required. Therefore, the development of
catalytic protocols for the carboxylation of allenamide with
Supporting information and ORCID(s) for the author(s) for this article can
be found under http://dx.doi.org/10.1002/chem.201601162.
Chem. Eur. J. 2016, 22, 8547 – 8552
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CO , which enables this carboxylation reaction at low catalyst
2
loading, is of great importance.
In recent years, N-heterocyclic carbene (NHC)–copper com-
plexes have been proved to be excellent catalysts for the car-
boxylation of various organic compounds with CO to give the
2
[1m,3c,g,i,l,18,19]
corresponding carboxylic acids or their derivatives.
As part of our studies on the use of CO as a building block for
2
organic synthesis, we recently reported the NHC-copper-cata-
lyzed alkylative carboxylation of ynamides with CO and di-
2
alkylzinc reagents, which afforded the corresponding a,b-dehy-
[3l]
dro-a-amino acid derivatives. Herein, we report the use of
(IPr)CuCl] as a catalyst for the alkylative carboxylation of allen-
[
Scheme 2. Hydromethylation and methylative carboxylation of 1a by cata-
lytic methylative zincation.
amides with CO and dialkylzinc reagents, which provides a,b-
2
dehydro-b-amino acid ester derivatives with well-controlled
regio- and stereoselectivity.
by KOtBu (20 mol%) (Scheme 2, Eq. (1)). The reaction of allen-
amide 1a (1.0 mmol) and Me Zn (1.5 mmol) was carried out in
2
Results and Discussion
the presence of a catalytic amount of [(IPr)CuCl] (5 mol%) in
THF at room temperature for 12 h. The reaction smoothly pro-
ceeded to afford the enamide 3-Me in 85% yield after the re-
Our initial concept is shown in Scheme 1. We speculated that
the alkylative carboxylation of allenamides might be achieved
by the combination of two fundamental processes. Namely,
action mixture was quenched with 10% aq. HCl (Scheme 2, Eq.
1
(
2)). The H NMR analysis and NOE experiments on 3-Me con-
firmed that the methyl group was introduced to the g-position
of 1a in a regioselective manner and that the CÀC double
bond in 3-Me adopted an (E)-configuration. When the reaction
mixture was quenched with D O, the enamide [D]3-Me was
2
obtained in 83% yield (D content, >95%). These results
strongly suggest the formation of an alkenylzinc species 2
through carbozincation of the allenyl moiety of 1a.
Scheme 1. Our strategy for carboxylation of allenamides by carbometalation.
the carbometalation of allenamide A would give the alkenyl
metal intermediate B, and the subsequent nucleophilic addi-
These positive outcomes encouraged us to perform the alky-
lative carboxylation reaction in a stepwise one-pot manner.
tion of B to CO would afford the corresponding alkylated car-
The reaction of allenamide 1a and Me Zn in the presence of
2
2
boxylate C, bearing an a,b-dehydro-b-amino acid skeleton. We
began our studies with screening catalysts, alkylating reagents,
and reaction conditions for the first step, that is, carbometala-
tion of allenamides, because a general and efficient catalytic
system for this reaction has not been reported.
[(IPr)CuCl] (5 mol%) in THF was carried out at room tempera-
ture for 12 h and the resulting mixture was subsequently treat-
ed with CO (1 atm.) at room temperature for 24 h. After hy-
2
drolysis of the reaction mixture and treatment of the obtained
crude material with CH N , the methylative carboxylation prod-
2
2
We recently demonstrated that [(IPr)CuCl] (IPr=1,3-bis(2,6-
diisopropenyl)imidazol-2-ylidene) could act as an efficient cata-
lyst for the carbozincation of ynamides with dialkylzinc re-
uct 4a-Me was isolated in 82% yield (Scheme 2, Eq. (3)). Again,
H NMR NOE studies on 4a-Me indicated that the carboxyla-
tion process occurred with reten-
tion of the configuration of the al-
kenyl moiety in a stereospecific
manner (Figure 1).
1
[
3l]
agents. Fortunately, in our initial study, we found that it
could also be used for allenamide substrates. First, we exam-
ined the carbozincation of 1a, which can be easily prepared
[17,20]
from propargyl amide 1a’
through isomerization catalyzed
We then attempted the methyla-
tive carboxylation of 1a in a one-
step, one-pot manner by simply
Figure 1. NOE of 4a-Me.
mixing allenamide 1a, Me Zn
2
(
(
1.5 mmol),
and
[(IPr)CuCl]
5 mol%) in THF under an atmosphere of CO (1 atm.) for 24 h
2
at room temperature. After hydrolysis and esterification, the
desired methylative carboxylation product 4a-Me was ob-
tained in 83% yield (Scheme 3), essentially the same as for the
aforementioned stepwise method.
The effect of various Cu catalysts for methylative carboxyla-
tion of allenamide 1a was studied and selected results are
summarized in Table 1. When CuCl was used in place of
[
(IPr)CuCl], the reaction did not proceed to afford either the
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Table 2. One-pot alkylative carboxylation of various cyclic allenamides
[a]
with various dialkylzinc reagents.
Scheme 3. One-pot method for methylative carboxylation of allenamide 1a.
Table 1. The effect of NHC–Cu complexes for the methylative carboxyla-
o
[b]
Entry
Allenamide 1
R
2
Zn
T [ C]
Product 2, yield [%]
[a]
tion of allenamide 1a.
1
2
1a
1a
Et
Bu
2
Zn
Zn
rt
rt
4a-Et (R=Et), 85
4b-Bu (R=Bu), 96
2
[b]
Entry
[Cat.]
Product, 4a-Me (yield,%)
[
c]
c]
1
2
3
4
5
6
without catalyst
CuCl
[(ICy)CuCl]
[(SIMes)CuCl]
[(SIPr)CuCl]
[(IMes)CuCl]
–
–
[
3
4
5
6
1b
1b
1b
1b
Me
Me
2
Zn
Zn
rt
4b-Me (R=Me), 72
4b-Me, 82
4b-Et, 79
2
50
50
50
trace
trace
49
Et
Bu
2
Zn
Zn
2
4b-Bu, 78
65
[
1
(
a] Reaction conditions: The reactions were carried out with allenamide
a (0.40 mmol), Me Zn (0.60 mmol) and catalyst (0.021 mmol) in THF
2.0 mL) under CO (1 atm., closed). [b] Isolated yields. [c] Starting material
2
2
7
8
9
1c
1c
1c
1c
Me
Me
Zn
Zn
rt
4c-Me (R=Me), 75
4c-Me, 81
4c-Et (R=Et), 84
4c-Bu (R=Bu), 77
2
was recovered quantitatively.
2
50
50
50
Et
Bu
2
Zn
Zn
1
0
2
desired methylated or carboxylated product (Table 1, entry 2)
with unreacted 1a recovered. The use of [(ICy)CuCl] or
[
(SIMes)CuCl] gave trace amounts of desired product (entries 3
1
1
13
1
2
1d
1d
1d
Me
Et Zn
Bu Zn
2
Zn
50
50
50
4d-Me (R=Me), 80
4d-Et (R=Et), 82
4d-Bu (R=Bu), 72
and 4). When either [(SIPr)CuCl] or [(IMes)CuCl] were used as
catalysts, the carboxylated product 4a-Me was obtained in 49
and 65% yields, respectively, along with the recovery of 25
and 20% of the starting material 1a (entries 5 and 6). From
these results it was found that [(IPr)CuCl] was the best catalyst
for methylative carboxylation of 1a.
2
2
14
15
16
1e
1e
1e
Me
Et Zn
Bu Zn
2
Zn
50
50
50
4e-Me (R=Me), 82
4e-Et (R=Et), 78
4e-Bu (R=Bu), 76
2
The reactivity of other dialkylzinc reagents, such as Et Zn
2
2
and Bu Zn, was investigated and both were found to be suita-
2
ble alkylating reagents in the current protocol despite contain-
ing b-hydrogen atoms. When Et Zn was used in the alkylative
2
carboxylation of 1a, the desired ethylative carboxylation took
place to give 4a-Et in 85% yield (Table 2, entry 1) without any
evidence of b-hydride elimination. These results contrast the
previously reported carboxylation reaction of alkynes and al-
17
18
1 f
1 f
1 f
Me Zn
50
50
50
4 f-Me (R=Me), 61
4 f-Et (R=Et), 78
4 f-Bu (R=Bu), 76
2
Et
Bu
2
Zn
Zn
19
2
[a] Reaction conditions: The reactions were carried out with allenamide
(0.40 mmol), R Zn (0.60 mmol), and [(IPr)CuCl] (0.021 mmol) in THF
2.0 mL) under CO (1 atm., closed). [b] Isolated yield.
1
(
2
lenes with Et Zn or other ethylmetal reagents in which b-hy-
2
2
dride elimination was observed to give hydrogenative carboxy-
[3d,h,7e]
lation products exclusively.
In the same manner, Bu Zn
2
could be used as the alkylating agent under similar conditions
to give the butylative carboxylation product 4a-Bu in 82%
yield (entry 2).
82% when the reaction was carried out at 508C (entry 4).
When the reactions of 1b with Et Zn and Bu Zn were per-
2
2
To expand the substrate scope of the present protocol, we
next examined the reaction of allenamides bearing phenyl- or
benzyl-substituted oxazolidinone moieties because such chiral
Evans’ oxazolidinone groups may provide potential applica-
formed at 508C, the corresponding alkylative carboxylation
products 4b-Et and 4b-Bu were obtained in 79 and 78%
yields, respectively (entries 5 and 6). The alkylative carboxyla-
tion of the related chiral allenamide 1c bearing a benzyl sub-
stituent could also be achieved in the same way. The methyla-
[21]
tions in asymmetric synthesis. The methylative carboxylation
of allenamide 1b with Me Zn and CO proceeded smoothly at
tive carboxylation of allenamide 1c with Me Zn at room tem-
2
2
2
room temperature to give the carboxylated product 4b-Me in
2% yield (Table 2, entry 3). The yield could be improved to
perature gave the desired product 4c-Me in 75% yield
(entry 7). When the reaction was carried out at 508C, the yield
7
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of 4c-Me was improved to 81% (entry 8). In addition, Et Zn
zincation process. While the reaction of 1h with Et Zn and
2
2
and Bu Zn could be used under similar reaction conditions and
Bu Zn gave the desired products in moderate yields (entries 5–
2
2
the ethylative and butylative carboxylation products 4c-Et and
6) with complete consumption of 1h, the reaction using
4
c-Bu were obtained in 84 and 77% yields, respectively (en-
Me Zn gave 4h-Me in 61% yield along with the recovery of
2
tries 9 and 10).
15% of the unreacted 1h (entry 4). Again, there was no im-
provement on the yield of 4h-Me after heating at 508C. Lastly,
we examined the reactions of the acyclic allenamide 1i, which
contains a benzyl carbamate moiety. This allenamide 1i also
underwent alkylative carboxylation with Me Zn, Et Zn, and
Allenamides bearing lactam moieties were also tolerated
under the reaction conditions. Pyrrolidone 1d cleanly under-
went methylative carboxylation to afford 4d-Me in 80% yield
at 508C with well-controlled regio- and stereoselectivity
2
2
(
Table 2, entry 11) despite the presence of relatively acidic pro-
Bu Zn to afford the corresponding alkylative carboxylation
2
tons a- to the amide group. The ethylative or butylative car-
boxylation of 1d by using Et Zn or Bu Zn also worked well to
products 4i-Me, 4i-Et, and 4iBu in 44, 64, and 52% yields, re-
spectively (entries 7–9).
2
2
afford the corresponding alkylative carboxylation product 4d-
Et (82%) or 4d-Bu (81%) (entries 12 and 13). Allenamides 1e
and 1 f bearing piperidinone and e-caprolactam units were
also suitable substrates for the alkylative carboxylation and the
desired products were obtained in good yields when the reac-
tions were performed at 508C (entries 14–19).
A possible catalytic cycle for the alkylative carboxylation of
allenamide 1a with CO is shown in Scheme 4. First, the dia-
2
Next, we examined the alkylative carboxylation of acyclic al-
lenamides and some selected results are reported in Table 3.
The reaction of tert-butoxycarbonyl (Boc)-protected acyclic al-
lenamide 1g with Me Zn gave methylative carboxylation prod-
2
uct 4g-Me in 41% yield at room temperature (Table 3, entry 1)
and 20% of starting allenamide substrate was recovered. At-
tempts to improve the yield of 4g-Me by heating at 508C
were unsuccessful. When Et Zn and Bu Zn were used, the alky-
2
2
lative carboxylation products 4g-Et and 4g-Bu was obtained in
[22]
6
4 and 55% yields, respectively (entries 2 and 3). At present,
the detail is not clear; however, the lower yield when Me Zn
2
was used could be due to stronger coordination of the less
bulky Me Zn to the Boc group which may inhibit the methyl-
2
Scheme 4. Possible catalytic cycle.
Table 3. Alkylative carboxylation of acyclic allenamides with various dia-
lkylzinc reagents.
lkylzinc reagent would react with the NHC-copper halide pre-
catalyst to form the alkylcopper species A. The amide carboxyl
group could work as a directing group (B) leading to the regio-
selective addition of the alkylcopper intermediate A to the al-
lenyl moiety of 1a to form the alkenylcopper species C. In this
catalytic system, transmetalation between the copper species
C and zinc species C’ would be a reversible process. Although,
the possibility of the direct nucleophilic carboxylation of C’
[a]
Entry
Allenamide 1
R
2
Zn
Product 4-R yield [%]
[
23]
with CO cannot be ruled out, the carboxylation would pref-
2
1
2
3
1g
1g
1g
Me
Et Zn
Bu Zn
2
Zn
4g-Me (R=Me), 41
4g-Et (R=Et), 64
4g-Bu (R=Bu), 55
erentially take place at the alkenyl copper species C by the in-
2
sertion of CO into the copper–carbon bond of C to form the
2
2
copper carboxylate intermediate D. The catalytically active spe-
cies would then be regenerated by the transmetalation of D
with dialkylzinc reagent, releasing the zinc carboxylate E.
During workup, the carboxylate E was hydrolyzed to give a car-
boxylic acid and the esterification of the resulting acid gives
the corresponding ester 4a-R.
It is also noteworthy that the methylative carboxylation reac-
tion starting from the transformation of propargyl amide 1a’
to allenamide 1a could be performed in one pot. First, the re-
action of 1a’ and KOtBu (20 mol%) was carried out at room
4
5
6
1h
1h
1h
Me
Et Zn
Bu Zn
2
Zn
4h-Me (R=Me), 61
4h-Et (R=Et), 72
4h-Bu (R=Bu), 66
2
[
b]
b]
2
[
7
8
9
1i
1i
1i
Me
Et Zn
Bu Zn
2
Zn
4i-Me (R=Me), 44
4i-Et (R=Et), 64
4iBu (R=Bu), 52
2
2
temperature to isomerize 1a’ to 1a. After 6 h, Me Zn
2
[
2
a] Isolated yield. [b] After 24 h the flask was refilled with CO .
(
1.5 equiv) and [(IPr)CuCl] (5 mol%) were added and the result-
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1
-nitrosoguanidine (ca. 70 mg, ca. 200 mmol) and 40% aq. KOH
(
0.5 mL) at 08C according to the standard procedure. The comple-
tion of the esterification was confirmed by a TLC analysis. After
evaporation of the solvent, the crude material was purified by
silica gel column chromatography (n-hexane/ethyl acetate=4:1) to
afford 4a-Me (colorless oil, 70.1 mg, 83%).
Methyl (Z)-2-ethyl-3-(2-oxooxazolidin-3-yl)acrylate (4a-Me)
À1
IR (film): n˜ =2921, 2850, 1762, 1709, 1365, 1296, 1140, 1084 cm
;
1
H NMR (500 MHz, CDCl ): d=6.82 (s, 1H), 4.40 (t, J=8.0 Hz, 2H),
3
Scheme 5. One-pot isomerization/alkylative carboxylation from propargyl
amide 1a’.
3.90 (t, J=8.0 Hz, 2H), 3.76 (s, 3H), 2.33 (q, J=7.5 Hz, 2H),
13
1.07 ppm (t, J=7.5 Hz, 3H); C NMR (125 MHz, CDCl ): d=167.6,
3
1
56.7, 129.6, 115.6, 62.7, 51.6, 44.5, 26.7, 14.3 ppm; HR MS (ESI):
+
calcd for C H NNaO [M+Na]C : 222.0736; found: 222.0734.
9
13
4
ing mixture was treated with CO (1 atm.) at room temperature
2
for 24 h. After the similar workup and purification processes,
methylative carboxylation product 4a-Me was isolated in 68% Acknowledgements
yield (Scheme 5). By the similar one-pot protocol, the reaction
using Et Zn and Bu Zn also provided 4a-Et and 4a-Bu in 62
This work was partly supported by a Grant-in-aid for Scientific
2
2
and 60% yields, respectively.
Research (S) (no. 26220802, to H.Z.) and (C) (no. 26460029, to
M.T.) from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. We are grateful to Ms. A. Karube for
high-resolution mass spectroscopic analysis.
Conclusion
By using an NHC–copper catalyst, we have developed, for the
first time, the highly efficient regio- and stereoselective alkyla-
Keywords: allenamides · carbon dioxide · carboxylation ·
copper · zinc
tive carboxylation of allenamides with CO and dialkylzinc re-
2
agents, which afforded various a,b-dehydro-b-amino acid ester
derivatives in good yields. This protocol possesses several syn-
thetic advantages, such as mild reaction conditions, easy one-
[1] Selected reviews: a) T. Sakakura, J. C. Choi, H. Yasuda, Chem. Rev. 2007,
1
07, 2365–2387; b) Z. Hou, T. Ohishi, in Comprehensive Organometallic
pot operation, 100% regio- and stereoselectivity, use of CO as
Chemistry III, Vol. 10 (Eds.: R. H. Crabtree, D. M. P. Mingos, I. Ojima),
Elsevier, Oxford, 2007; pp. 537–555; c) A. Correa, R. Martín, Angew.
Chem. Int. Ed. 2009, 48, 6201–6204; Angew. Chem. 2009, 121, 6317–
2
a building block, and formation of two CÀC bonds in a single
catalytic system. These features make the present method
practically useful and attractive in synthetic organic chemistry.
It is also worth noting that the formation of an alkenyl zinc in-
termediate such as 2 through catalytic methylative zincation of
allenamides has played an important role in the present trans-
formation. Such catalytic carbometalation may also find appli-
cations for other useful chemical transformations.
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VCH, Weinheim, 2010; e) New and Future Developments in Catalysis:
Activation of Carbon Dioxide (Ed.: S. L. Suib), Elsevier, Amsterdam,
2013; f) S. N. Riduan, Y. G. Zhang, Dalton Trans. 2010, 39, 3347–3357;
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